Soil salinization and alkalization severely limit agricultural sustainability in alpine regions. The integrated responses of oats to complex saline–alkali stress, covering phenotypic, physiological, and metabolomic alterations, remain poorly elucidated. This study aimed to clarify the synergistic adaptive mechanisms of oats with differential saline–alkali tolerance. Three oat varieties with distinct tolerance levels, including the tolerant cultivar Meida, the moderately tolerant cultivar Qingtian No. 2, and the sensitive cultivar Qinghai Sweet Oat, were treated with a 150 mmol·L⁻¹ mixed saline–alkali solution (Na₂SO₄:NaCl:NaHCO₃ = 2:1:1, pH 8.65) to simulate the natural alpine soil environment of Qinghai Province. Compound saline–alkali stress markedly inhibited oat growth and aggravated cellular oxidative damage. The tolerant cultivar effectively alleviated stress injury by improving antioxidant enzyme activities and accumulating osmoprotectants. A total of 396 core differential metabolites were screened in this study, and flavonoid biosynthesis was identified as a conserved core pathway for oat stress resistance. Tolerant oat varieties simultaneously activated defense responses and energy metabolism to adapt to stress conditions. In contrast, sensitive varieties only depended on basal metabolic adjustments to cope with stress. These findings clarify the differential adaptive strategies of oats under saline–alkali stress. They provide key metabolic marker resources and a solid theoretical basis for the breeding and cultivation of salt–alkali-tolerant oats in alpine saline–alkali land.